Optical communication system

ABSTRACT

An optical communication system has a plastic optical fiber (POF) and an optical communication module. The POF has a spherical end surface, and light emitted from the spherical end surface has an NA of 0.35 or lower. The POF is installed in the module such that a light receiving surface of a light receiving element (PD) is at a distance, d, from an apex of the spherical end surface. The distance, d, is within a range of 0&lt;d≦r*D/(n−n 1 ) when a PD diameter is not larger than D, and within a range of D≦d≦r*D/(n−n 1 ) when the PD diameter is larger than D, where D is a diameter of the POF, r*D is a radius of curvature of the spherical end surface, n is a refractive index of a core of the POF, and n 1  is a refractive index of a substance between the spherical end surface and the PD.

This application is the U.S. National Phase entry of InternationalApplication No. PCT/JP2003/010543, filed on Aug. 21, 2003, whichdesignated the United States. PCT/JP2003/010543 claims priority toJapanese Patent Application No. 2002-241982, filed on Aug. 22, 2002. Theentire contents of these applications are incorporated by referenceherein.

BACKGROUND OF THE INVENTION

The present invention relates to an optical communication system capableof transmitting and receiving an optical signal by means of an opticalfiber and relates, in particular, to an optical communication systemapplicable to domestic communication, communication between electronicequipment, LAN (Local Area Network) and so on using a plastic opticalfiber as a transmission medium.

An optical communication system using an optical fiber has atransmission system on one end side of a signal transmission path of theoptical fiber and has a reception system on the other end side of thetransmission path. The transmission system includes a light source(light emitting element) of, for example, a light emitting diode, asemiconductor laser or the like and makes a signal light, which isobtained by making the light emitting source emit light under control,incident on the optical fiber. On the other hand, the reception systemincludes a light receiving element of, for example, a photodiode or thelike, and the light receiving element receives the signal light emittedfrom the optical fiber and converts the signal into an electricalsignal.

The performance of the optical communication system largely depends onthe transmission efficiency of the signal light. Moreover, thetransmission efficiency is determined mainly by the transmissionefficiency of the optical fiber itself, the coupling efficiency of thelight emitting source to the optical fiber and the coupling efficiencyof the optical fiber to the light receiving element.

The reception systems of the conventional optical communication systemsare roughly categorized into two types: a type that receives outgoinglight from the optical fiber directly on the light receiving element anda type that receives the outgoing light by collecting the light via anoptical system such as f a lens or the like arranged between the opticalfiber and the light receiving element.

Such a scheme of optical coupling between the optical fiber and thelight receiving element is widely used for a quartz fiber whose corediameter is on the order of micrometers. However, a problem occurs inthe case of a plastic optical fiber whose core diameter is on the orderof millimeters. The plastic optical fiber is the optical fiber that hasrecently attracted attention for home network and the like. The plasticoptical fiber has a large fiber diameter of 0.5 to 2 mm and is easy toconnect, whereas it has a problem that the coupling efficiency to thereceiver is reduced because of a large aperture. Normally, the diameterof light reception of the light receiving element used for optical fibercommunication is several hundreds of micrometers to one hundredmicrometers, and accordingly, there is no problem in the case of anoptical fiber of a small core diameter. However, in the case of, forexample, a plastic optical fiber having an aperture of 1 mm, it isdifficult to collect light to a size smaller than the size of the lightsource even when a lens or the like is used. Particularly, the diameterof light reception is required to be reduced in relation to capacity asthe transmission rate is increased, and therefore, a reduction in thecoupling efficiency, i.e., the reception efficiency occurs.

To solve the problem, an optical communication system, which has astructure for coupling the optical fiber with the light receivingelement as shown in FIG. 25, is known. In this optical communicationsystem, an optical guide 101, which has an optical guide path 102enclosed by a highly reflective reflection surface 103, is placedbetween an optical fiber 104 and a light receiving element 105, and asignal light emitted from the optical fiber 104 is guided to the lightreceiving element 105 by the optical guide 101. With this arrangement,the optical fiber 104 is optically coupled with the light receivingelement 105 with high efficiency, and even outgoing light from anoptical fiber of a large core diameter such as the plastic optical fibercan efficiently be collected to a photodiode of a small aperture (referto JP 10-221573 A, paragraph 0008 and FIGS. 1 and 3).

In the case of the structure as shown in FIG. 25, when the numericalaperture (NA) of the light emitted from the optical fiber is changed,and particularly enlarged, there is a drawback that the outgoing light106 tends to return to the optical fiber 104 side as shown in FIG. 26,so that the coupling efficiency is reduced. Moreover, the structure hasa manufacturing problem that the aspect ratio of the hole depth to theaperture is large, so that it is difficult to uniformly deposit areflection coating.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an opticalcommunication system capable of efficiently achieving optical couplingof an optical fiber of a large aperture like the plastic optical fiberwith a light receiving element of a small aperture with a simpleconstruction.

An optical communication system according to an aspect of the presentinvention comprises:

an optical fiber having a spherical end surface at at least one endthereof, wherein radiant light emitted from the spherical end surfacehas a numerical aperture of not larger than 0.35; and

an optical communication module which has a light receiving element andreceives the radiant light emitted from the spherical end surface of theoptical fiber, wherein,

when the one end of the optical fiber is inserted in a prescribedportion inside the optical communication module, a light receivingsurface of the light receiving element is located at a distance, d, froman apex of the spherical end surface of the optical fiber, and

assuming that a diameter of the optical fiber is D, a radius ofcurvature, R, of the spherical end surface is r*D, a refractive index ofa core of the optical fiber is n, and a refractive index of a substancethat exists between the spherical end surface of the optical fiber andthe light receiving element is n1, then the distance, d, is:

within a range of 0<d≦r*D/(n−n1) when a diameter of the light receivingelement is not larger than D, and

within a range of D≦d≦r*D/(n−n1) when the diameter of the lightreceiving element is larger than D.

It is to be noted that the “diameter of the optical fiber” hereindicates the core diameter. In the case of an SI type plastic opticalfiber, the cladding portion accounts for only two percent of the totaldiameter of the fiber and therefore, the diameter of the optical fiberbecomes approximately equal to the core diameter.

By forming the end surface of the optical fiber into the spherical endsurface and arranging the light receiving element so that the distance,d, is within the range, the reception coupling efficiency can beimproved to two times or more at maximum in comparison with the casewhere the end surface of the optical fiber is a flat surface.

The end surface of the fiber processed into a spherical shape isconsidered to be equivalent to a structure in which a plano-convex lensthat has a convex surface in a direction in which the light from thefiber is emitted is attached to a flat surface of the fiber. A valueobtained from r*D/(n−n1), i.e., R/(n−n1) indicates a focal distance, f,in a space filled with a substance of a refractive index n1, of theplano-convex lens of which the radius of curvature is R and therefractive index is n.

FIGS. 3A through 3C are schematic views showing the relation between thespread (far-field pattern: FFP) of outgoing light L from the opticalfiber and the radius of curvature, R (=r*D), of the spherical endsurface 11 provided by processing the optical fiber end surface, therelation having been obtained from the simulation carried out in the air(n1=1) with the plastic optical fiber 1 whose core is made of PMMA(refractive index≈1.5).

If a multi-mode optical fiber, in particular, an SI (Step-Index) typemulti-mode optical fiber is cut along a plane perpendicular to the axialdirection of the fiber, then its near-field pattern can be regarded as asurface light source of uniform intensity. Moreover, the orientationdistribution of the outgoing light from each of points into which thesurface light source of the uniform intensity is subdivided, is Gaussiandistribution.

As is apparent from FIGS. 3A through 3C, the position where the outgoinglights are collected differs depending on the radius of curvature of thespherical end surface of the fiber. FIG. 3A shows a case where theradius of curvature, R, of the spherical end surface is two times thefiber diameter, D, i.e., R=2*D, and the light collection position islocated at the distance of 4D from the apex of the spherical endsurface. FIG. 3B shows a case where the radius of curvature, R, of thespherical end surface is one and half times the fiber diameter, D, i.e.,R=1.5*D, and the light collection position is located at the distance of3D from the apex of the spherical end surface. FIG. 3C shows a casewhere the radius of curvature, R, of the spherical end surface is equalto the fiber diameter, D, i.e., R=D, and the light collection positionis located at the distance of 2D from the apex of the spherical endsurface.

The focal distance, f, of the plano-convex lens in the air is expressedby f=R/(n−1) from the above, and the simulation results shown in FIGS.3A through 3C almost coincide with the focal distance, f, in the airwhen the refractive index of the plano-convex lens is set to 1.5.

That is, according to the present invention, the light receiving surfaceof the light receiving element is placed within the focal distance, f,when the outgoing light from the optical fiber has a small numericalaperture (NA) of 0.35 or below. However, the results of variousexperiments conducted by the inventors have indicated that when thediameter of the light receiving element is larger than the diameter, D,of the optical fiber, only the coupling efficiency equivalent to that ofthe optical fiber of which the end surface is flat can be obtained untilthe distance, d, exceeds D. That is why the distance, d, is made notsmaller than D. Therefore, the radiant light emitted from the opticalfiber end surface is collected by the plano-convex lens effect and madeincident on the light receiving element before spreading again.Consequently, the coupling efficiency to the light receiving element isimproved and becomes higher than when the end surface of the opticalfiber is flat. Furthermore, an optical guide as required in the priorart case is not used, and therefore, the optical communication module iseasily manufactured by that much.

The optical fiber of which the NA of the outgoing light is 0.35 is usedmainly for high speed transmission at a transmission rate of about 200to 622 Mbps. Normally, it is necessary to reduce the diameter of thelight receiving element in relation to the capacity as the transmissionrate is increased. Moreover, the higher the transmission rate, thesmaller the structural NA of the optical fiber to be used. In accordancewith the above, the NA of the light emitted from the optical fiber alsobecomes small. The present invention is effective particularly when thediameter of the light receiving element is small and a plastic opticalfiber of a small optical fiber NA is used, i.e., at the time of highspeed transmission of several hundreds of Mbps with a plastic opticalfiber used.

In the optical communication system of the present invention, thecommunication module may have, in addition to the light receivingelement, a reception optical system that guides the light emitted fromthe spherical end surface of the optical fiber to the light receivingelement. In this case, not the light receiving surface of the lightreceiving element but a center position of the reception optical systemis to be arranged at the distance, d, from the spherical end surface ofthe optical fiber according to the size of the reception optical systemas follows. That is, the reception optical system is arranged so thatthe distance, d, from the spherical end surface of the optical fiber tothe center position of the reception optical system is:

within the range of 0<d≦r*D/(n−n1) when the size of the receptionoptical system is not larger than D, and

within the range of D≦d≦r*D/(n−n1) when the size of the receptionoptical system is larger than D.

Examples of the reception optical system include a light refractingmember, such as a prism, a lens or the like, which is formed of asubstance of a refractive index different from that of air, and a lightreflecting member such as a mirror or the like. When a transparentmolded piece or the like of a refractive index different from that ofair is formed on the light receiving element, even such a molded pieceis treated as a reception optical system in the present application.

Herein, the “center position of the reception optical system” means aprincipal point on the incidence side upon the reception optical systemof the principal ray of light from the optical fiber.

Moreover, the “size of the reception optical system” is defined as thediameter of a portion of the system that optically collect the lightwhen the shape of the reception optical system is circular (e.g.,condenser lens), and is defined as a typical dimension of a portion ofthe system that optically collect the light when the shape is notcircular (e.g., prism).

According to various simulation results, the distance, d, shouldpreferably be:

within a range of 0<d≦2D when the diameter of the light receivingelement is not larger than D, and

within a range of D≦d≦2D when the diameter of the light receivingelement is larger than D.

Moreover, the present invention is more effective when the diameter ofthe light receiving element (or the size of the reception optical systemwhen the reception optical system is provided) is not larger than thediameter, D, of the optical fiber. This is because the effect ofimprovement in the coupling efficiency upon the optical fiber having aflat end surface is more remarkable than in the case where the diameterof the light receiving element (or the size of the reception opticalsystem when the reception optical system is provided) is larger than thediameter, D, of the optical fiber.

An optical communication system according to another system comprises:

an optical fiber having a spherical end surface at at least one endthereof, wherein radiant light emitted from the spherical end surfacehas a numerical aperture of 0.4–0.6 inclusive; and

an optical communication module which has a light receiving element andreceives the radiant light emitted from the spherical end surface of theoptical fiber, wherein,

when the one end of the optical fiber is inserted in a prescribedportion inside the optical communication module, a light receivingsurface of the light receiving element is located at a distance, d, froman apex of the spherical end surface of the optical fiber, and

assuming that a diameter of the optical fiber is D, then the distance,d, is:

within a range of 0<d<2D when a diameter of the light receiving elementis not larger than D, and

within a range of 0.5D<d<2D when the diameter of the light receivingelement is larger than D.

The numerical aperture of the outgoing light within the range of from0.4 to 0.6, in particular, the numerical aperture of 0.5 is used formedium speed transmission at a transmission rate of about 20 to 100Mbps.

In this optical communication system, the communication module may have,in addition to the light receiving element, a reception optical systemthat guides the light emitted from the spherical end surface of theoptical fiber to the light receiving element. In this case, not thelight receiving surface of the light receiving element but a centerposition of the reception optical system is to be arranged at thedistance, d, from the spherical end surface of the optical fiberaccording to the size of the reception optical system as follows. Thatis, the reception optical system is arranged so that the distance, d,from the spherical end surface of the optical fiber to the centerposition of the reception optical system is:

within the range of 0<d<2D when a size of the reception optical systemis not larger than D, and

within the range of 0.5D<d<2D when the size of the reception opticalsystem is larger than D.

By forming the end surface of the optical fiber into a spherical endsurface and arranging the reception optical system such that thedistance, d, is within the range, the reception coupling efficiency canbe improved up to about 1.7 times at maximum as large as in the casewhere the end surface of the optical fiber is a flat surface.

The definitions of the “center position of the reception optical system”and the “size of the reception optical system” are as described above.

According to various simulation results, the distance, d, shouldpreferably be:

within a range of 0<d≦1.5D when the diameter of the light receivingelement is not larger than D, and

within a range of D≦d≦1.5D when the diameter of the light receivingelement is larger than D.

Similarly to the case where the numerical aperture is not larger than0.35, the diameter of the light receiving element (the size of thereception optical system when the reception optical system is provided)should preferably be not larger than the diameter, D, of the opticalfiber. This is because the effect of improvement in the couplingefficiency upon the optical fiber having a flat end surface is moreremarkable than in the case where the diameter of the light receivingelement (or the size of the reception optical system when the receptionoptical system is provided) is larger than the diameter, D, of theoptical fiber. Therefore, if the present invention is utilized, then asmall-sized reception optical system, which easily collects light on asmall light receiving element, can be arranged. In this case, thepresent invention is more effective in single-core two-waycommunication.

In one embodiment, each of the above-described optical communicationmodules further comprises, of a light emitting element and atransmission optical system, at least the light emitting element suchthat the optical communication module is able to transmit and receive asignal light via the optical fiber to and from a counterpart opticalcommunication module in a single-core two-way communication scheme. Thisarrangement is effective from the viewpoint of combined arrangement ofthe transmission system and the reception system since the lightreceiving element and the reception optical system can be reduced insize.

As described above, when the plastic optical fiber is used, a commonfiber diameter is from 0.5 mm to 2 mm. Among others, a fiber having adiameter of 1 mm is widely used from the viewpoint of easiness of use,i.e., easiness of connection and suppression of modal dispersion. On theother hand, the transmission rate of high speed communication generallyused for the plastic optical fiber is from 100 Mbps to 622 Mbps, and thediameter of a photodiode (also referred to as a “PD diameter” below)suitable for that transmission rate is not larger than 0.5 mm and moreprecisely from 0.3 mm to 0.5 mm.

In one embodiment, an optical fiber that has a diameter, D, of 1 mm anda small-sized photodiode that has a diameter of not larger than 0.5 mm(e.g., 0.3 mm to 0.5 mm) and is able to cope with high speed are used incombination in either one of the communication systems. The combinationof such dimensions of the optical fiber and the light receiving elementis effective since the reception efficiency can largely be improved bythe application of the present invention in comparison with the fiber ofa flat end surface.

Moreover, in the case where the reception optical system is provided, ifthe optical fiber of which the diameter, D, is 1 mm is used, the size ofthe reception optical system should preferably be made 0.5 mm or smallerfor the same reasons as those described immediately before.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing the construction of an opticalcommunication system according to one embodiment of the presentinvention;

FIG. 2 is a view schematically showing the construction of an opticalcommunication system according to one embodiment of the presentinvention;

FIGS. 3A, 3B and 3C are diagrams for explaining the principle of thepresent invention;

FIG. 4 is a graph indicating the effects of the present invention, inwhich under the condition that the diameter of the light receivingelement is 0.5D and the fiber emission NA is 0.35, the receptioncoupling efficiency in the case where a fiber with a spherical endsurface is used is compared with that of the case where a fiber with aflat end surface is used;

FIG. 5 is a graph indicating the effects of the present invention, inwhich under the condition that the diameter of the light receivingelement is D and the fiber emission NA is 0.35, the reception couplingefficiency in the case where a fiber with a spherical end surface isused is compared with that of the case where a fiber with a flat endsurface is used;

FIG. 6 is a graph indicating the effects of the present invention, inwhich under the condition that the diameter of the light receivingelement is 1.5D and the fiber emission NA is 0.35, the receptioncoupling efficiency in the case where a fiber with a spherical endsurface is used is compared with that of the case where a fiber with aflat end surface is used;

FIG. 7 is a graph indicating the effects of the present invention, inwhich under the condition that the diameter of the light receivingelement is 0.5D and the fiber emission NA is 0.5, the reception couplingefficiency in the case where a fiber with a spherical end surface isused is compared with that of the case where a fiber with a flat endsurface is used;

FIG. 8 is a graph indicating the effects of the present invention, inwhich under the condition that the diameter of the light receivingelement is D and the fiber emission NA is 0.5, the reception couplingefficiency in the case where a fiber with a spherical end surface isused is compared with that of the case where a fiber with a flat endsurface is used;

FIG. 9 is a graph indicating the effects of the present invention, inwhich under the condition that the diameter of the light receivingelement is 1.5D and the fiber emission NA is 0.5, the reception couplingefficiency in the case where a fiber with a spherical end surface isused is compared with that of the case where a fiber with a flat endsurface is used;

FIG. 10 is a table in which the reception coupling efficiencies, shownin the graph of FIG. 4, for the optical fibers (emission NA=0.35) eachhaving a spherical end surface in comparison with the fiber having aflat end surface are classified under three groups;

FIG. 11 is a table in which the reception coupling efficiencies, shownin the graph of FIG. 5, for the optical fibers (emission NA=0.35) eachhaving a spherical end surface in comparison with the fiber having aflat end surface are classified under three groups;

FIG. 12 is a table in which the reception coupling efficiencies, shownin the graph of FIG. 6, for the optical fibers (emission NA=0.35) eachhaving a spherical end surface in comparison with the fiber having aflat end surface are classified under three groups;

FIG. 13 is a table in which the reception coupling efficiencies, shownin the graph of FIG. 7, for the optical fibers (emission NA=0.5) eachhaving a spherical end surface in comparison with the fiber having aflat end surface are classified under three groups;

FIG. 14 is a table in which the reception coupling efficiencies, shownin the graph of FIG. 8, for the optical fibers (emission NA=0.5) eachhaving a spherical end surface in comparison with the fiber having aflat end surface are classified under three groups;

FIG. 15 is a table in which the reception coupling efficiencies, shownin the graph of FIG. 9, for the optical fibers (emission NA=0.5) eachhaving a spherical end surface in comparison with the fiber having aflat end surface are classified under three groups;

FIG. 16 is a graph indicating the effects of the present invention, inwhich under the condition that the diameter of the light receivingelement is 0.5D and the fiber emission NA is 0.35, the receptioncoupling efficiency in the case where a fiber with a spherical endsurface is used is compared with that of the case where a fiber with aflat end surface is used;

FIG. 17 is a graph indicating the effects of the present invention, inwhich under the condition that the diameter of the light receivingelement is 0.5D and the fiber emission NA is 0.5, the reception couplingefficiency in the case where a fiber with a spherical end surface isused is compared with that of the case where a fiber with a flat endsurface is used;

FIG. 18 is a graph indicating the effects of the present invention, andshowing the PD diameter dependence of the reception coupling efficiency;

FIG. 19 is a graph indicating the effects of the present invention, andshowing comparison between the actually obtained values of the receptioncoupling efficiency and the values obtained by simulation;

FIG. 20 is a view schematically showing the construction of an opticalcommunication system employing the single-core two-way communicationscheme according to one embodiment of the present invention;

FIG. 21 is an enlarged schematic view showing part (neighborhood of theoptical fiber end surface) of the optical communication system of FIG.20;

FIG. 22 is an explanatory view showing the relation between thedimension and position of the optical system in the opticalcommunication system of the single-core two-way communication system andthe coupling of the transmission/reception light with the optical fiberend surface/reception optical system;

FIG. 23 is an explanatory view showing the relation between thedimension and position of the optical system in the opticalcommunication system of the single-core two-way communication system andthe coupling of the transmission/reception light with the optical fiberend surface/reception optical system;

FIG. 24 is an explanatory view showing the relation between thedimension and position of the optical system in the opticalcommunication system of the single-core two-way communication system andthe coupling of the transmission/reception light with the optical fiberend surface/reception optical system;

FIG. 25 is an explanatory view of prior art; and

FIG. 26 is a view for explaining the problem of the prior art shown inFIG. 25.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail below on the basis ofthe embodiments shown in the drawings.

(First Embodiment)

FIG. 1 schematically shows one example of the optical communicationsystem for carrying out one-way communication as an embodiment of theoptical communication system of the present invention. This opticalcommunication system includes an optical fiber 1 and a pair of opticalcommunication modules 2A and 2B for transmitting and receiving a signallight via the optical fiber 1. The optical communication module 2B has alight emitting element 22 constructed of a semiconductor laser device(LD) or a light emitting diode (LED) and functions as a transmissionmodule. On the other hand, the optical communication module 2A has alight receiving element 21 constructed of a photodiode (PD) andfunctions as a reception module. For the sake of simplicity, portionsthat have no direct relation to the invention, such as parts retainingthe light receiving and emitting elements and so on, are not shown inFIG. 1.

The optical fiber 1 is a plastic optical fiber whose core is made ofPMMA (refractive index: approximately 1.5), and both end surfaces areeach formed into a spherical end surface 11 that has a radius ofcurvature, R,. It is to be noted that only the end surface on thereception side may be formed into the spherical end surface 11.Moreover, the optical fiber 1 may be made of a plastic material otherthan PMMA. The spherical end surface 11 of the optical fiber can beformed by melting or polishing.

When the end portion of the optical fiber 1 is inserted into the opticalcommunication module 2A and placed in a prescribed location, the lightreceiving surface of the light receiving element 21 is located in aposition at a distance, d, from the apex of the spherical end surface 11of the optical fiber 1. The distance, d, has a value set in accordancewith the numerical aperture (also referred to as an “emission NA” below)of the radiant light emitted from the spherical end surface 11 of theoptical fiber 1 and the diameter (also referred to as a “PD diameter”below) of the photodiode 21 that is the light receiving element.

Concretely, in the case where the emission NA of the optical fiber 1 isnot larger than 0.35 used for high speed transmission at a transmissionrate of 200 to 622 Mbps, the distance, d, is set within a range of:0<d≦r*D/(n−n1)  (1)when the diameter of the light receiving element is not larger than D,andD≦d≦r*D/(n−n1)  (2)when the diameter of the light receiving element is larger than D. Inthe above expressions, D represents the diameter (core diameter) of theoptical fiber 1, r*D expresses the radius of curvature, R, of thespherical end surface 11 using D, n represents the refractive index ofthe core of the optical fiber 1, and n1 represents the refractive indexof the substance that exists between the spherical end surface 11 of theoptical fiber 1 and the light receiving element 21. In the case of thepresent embodiment, the substance that exists between the spherical endsurface 11 of the optical fiber 1 and the light receiving element 21 isair. Therefore, n1 is one. The refractive index of the core material ofPMMA (polymethyl is methacrylate) of the optical fiber 1 isapproximately 1.5 (treated as 1.5 here for calculation) Therefore, therelational expressions (1) and (2) can be rewritten as:0<d≦2r*D  (1′) andD≦d≦2r*D  (2′)respectively.

The expression (1′) indicates that the light receiving surface of thelight receiving element 21 is not in contact with the spherical endsurface 11 of the optical fiber 1 and not located away from thespherical end surface 11 of the optical fiber 1 by an amount in excessof a distance that corresponds to two times the radius of curvature ofthe spherical end surface 11. The expression (2′) indicates that thelight receiving surface of the light receiving element 21 is locatedaway from the spherical end surface 11 of the optical fiber 1 by anamount equal to or more than a distance that corresponds to the diameterof the optical fiber 1, but not in excess of the distance thatcorresponds to two times the radius of curvature of the spherical endsurface 11.

On the other hand, in the case where the emission NA of the opticalfiber 1 is about 0.5 (i.e., 0.4 to 0.6) used for medium speedtransmission at a transmission rate of about 100 to 200 Mbps, thedistance, d, is set within a range of:0<d≦2D  (3)when the diameter of the light receiving element is not larger than D,and0.5D<d<2D  (4)when the diameter of the light receiving element is larger than D.

FIGS. 4 through 6 are graphs showing the dependence of the receptioncoupling efficiency on end surface-to-receiver distance in the casewhere the emission NA of the optical fiber 1 is 0.35 in the opticalcommunication system that has the construction of FIG. 1, in comparisonwith the case of a fiber having a flat end surface, where each parameteris expressed by using the fiber diameter, D. More in detail, thereception coupling efficiency plotted on the vertical axis of the graphis a reception coupling efficiency obtained when the numerical apertureof outgoing light from the optical fiber, which light is defined by anintensity of 1/e² (≈0.135) when the fiber emission end surface is a flatsurface, is 0.35, the transmission rate is 200–500 Mbps, a semiconductorlaser (LD) is used as a light source, and a low NA and high speedcommunication grade plastic optical fiber (refractive index, n, of thecore is 1.5) is used as the transmission medium. The coupling efficiencyin the graphs is expressed in the form of a ratio to the couplingefficiency for the flat fiber end surface (i.e., a coupling efficiencyof one (“1”) means a coupling efficiency when the fiber end surface is aflat surface). The horizontal axis represents the endsurface-to-receiver distance (distance between the end surface and thereceiver) in the form of a ratio to the fiber diameter, D. Moreover, theradius of curvature, R, of the end surface and the PD diameter, whichare the parameters, are expressed by using the fiber diameter, D. FIGS.4, 5 and 6 show the case where the PD diameter is 0.5D, the case wherethe PD diameter is 1D and the case where the PD diameter is 1.5D,respectively. Moreover, the marks ♦, ▪ and ▴ represent the cases wherethe radius of curvature, R, of the end surface is 2D, 1.5D and D,respectively. It is to be noted that the “receiver” refers to thephotodiode 21 in this case.

Moreover, FIGS. 10 and 11 show the effects concerning the receptioncoupling efficiency shown in the graphs of FIGS. 4 through 6, tabulatedunder three groups, where “∘” indicates that the reception couplingefficiency is more than 1.01 times that of the fiber with the flat endsurface, “Δ” indicates that the reception coupling efficiency is 0.99 to1.01 times that of the fiber with the flat end surface, and “x”indicates that the reception coupling efficiency is less than 0.99 timesthat of the fiber with the flat end surface.

It can be seen from the graphs that the reception efficiency has beenimproved, as compared with the case of the fiber end surface being flat,until after a position corresponding to the focal position determined bythe radius of curvature, R (=r*D), of the spherical end surface shown inFIGS. 3A through 3C (in this case, because n=1.5 and n1=1,f=r*D/0.5=2r*D) is reached. It is to be noted that the effect is largerwhen the PD diameter is smaller. It can also be understood that when thePD diameter is 0.5D, the reception efficiency can be increased as theradius of curvature, R, of the end surface is smaller. In any case, whenthe PD diameter is not larger than the fiber diameter, D, the effect ofincreasing the reception efficiency is enjoyed from the position of thefiber end surface to the position that corresponds to the focalposition.

On the other hand, it can be understood that when the PD diameter islarger than the fiber diameter, D, there is an increase in the receptionefficiency from the position located away from the fiber end surface by1D to the position that corresponds to the focal position determined bythe radius of curvature, R (=r*D), of the spherical end surface.

FIGS. 7 through 9 show graphs similar to those of FIGS. 4 through 6,showing the coupling efficiency obtained when the fiber emission NA oflight from an optical fiber, which light is defined by an intensity of1/e² when the fiber emission end surface is a flat surface, is 0.5. Inthis case, however, the transmission rate was from 100 to 200 Mbps, anLED was used as a light source (light emitting element), and a plasticoptical fiber (refractive index, n, of the core is 1.5) with acommunication grade of an NA of about 0.5 was used as a transmissionmedium. FIGS. 7, 8 and 9 respectively show the case where the PDdiameter is 0.5D, the case where the PD diameter is 1D, and the casewhere the PD diameter is 1.5D.

FIGS. 13 through 15 show the effects regarding the reception couplingefficiency shown in the graphs of FIGS. 7 through 9, tabulated underthree groups, where “∘” indicates that the reception coupling efficiencyis more than 1.01 times that of the fiber with the flat end surface, “Δ”indicates that the reception coupling efficiency is 0.99 to 1.01 timesthat of the fiber with the flat end surface, and “x” indicates that thereception coupling efficiency is less than 0.99 times that of the fiberwith the flat end surface, similarly to FIGS. 10 through 12.

It is apparent from FIGS. 7 through 9 and FIGS. 13 through 15 that whenthe PD diameter is smaller than the fiber diameter, D, the receptionefficiency has been improved in a range from the neighborhood of thefiber end surface to the position of 2D, as compared with the case wherethe fiber end surface is the flat surface, although the effect ofimprovement in the reception efficiency in this case is smaller thanwhen the NA of the fiber emission defined by the intensity of 1/e² is0.35. Also, it is apparent that also in this case as well, the receptionefficiency can be increased as the radius of curvature, R, of the endsurface is smaller when the PD diameter is smaller than the fiberdiameter, D, (provided that the distance, d, is within 1D). Moreover, ithas been found that even when the PD diameter is larger than the fiberdiameter, D, there is an effect of improvement in the receptionefficiency as far as the distance is within the range of from theneighborhood of the fiber end surface D to the position of 2D althoughthe degree of improvement is reduced.

FIGS. 16 and 17 are graphs showing the distance dependence of thereception coupling efficiency with the radius of curvature, R, of thefiber spherical end surface 11 changed, as compared with that of thecase having the flat fiber end surface, where the diameter of the lightreceiving element 21 is 0.5D and the NA of fiber emission defined by theintensity of 1/e² when the fiber emission end surface is a flat surfaceis 0.35 and 0.5, respectively. It is apparent from these graphs thatwhen the fiber end surface is a spherical end surface, the receptionefficiency (coupling efficiency) can be increased within the distancerange defined by the expressions (1) and (3), in comparison with thecase where the fiber end surface is a flat surface. Above all, when theradius of curvature, R, of the spherical end surface 11 is D and thedistance, d, is in the neighborhood of 0.5D, the coupling efficiency islargely improved in comparison with the fiber with a flat end surfaceregardless of whether the NA of the emission from the optical fiber is0.35 or 0.5. Moreover, it can be said that with respect to one distance,d, the coupling efficiency is increased as the radius of curvature, R,of the end surface becomes smaller, i.e., as the curvature becomeslarger, as far as the distance, d, is within a prescribed range.

FIG. 18 is a graph in which the dependence of the reception efficiencyon the light receiving element diameter (PD diameter) is plotted usingthe distance, d, as a parameter in the case where the NA of the fiberemission, which is defined by the intensity of 1/e² when the fiberemission end surface is a flat surface, corresponds to 0.35. It isapparent from FIG. 18 that the arrangement is effective as the PDdiameter is smaller and, in particular, when the PD diameter is smallerthan the fiber diameter of 1D. Also, it can be seen that a higherreception efficiency is obtainable when the distance, d, is 1D than whenit is 1.5D, in the case where the PD diameter is smaller than the fiberdiameter of 1D and, in particular, when the fiber diameter is not largerthan approximately 0.9D. It is regarded as effective to set thedistance, d, to a value within 1D when the PD diameter is smaller thanthe fiber diameter, D.

FIG. 19 is a graph showing a comparison between calculated values andactual measurement results in the case where the NA of emission from theoptical fiber 1, which emission is defined by the intensity of 1/e² whenthe fiber emission end surface is a flat surface, corresponds to 0.35,the optical fiber diameter is 1 mm, the radius of curvature of the fiberspherical end surface 11 is 1.5 mm, and the PD diameter is 1 mm. Boththe calculated values and the actual measurement values almost coincidewith each other and exhibit the same tendency. It was confirmed that acoupling efficiency exceeding 30% was obtained when the endsurface-to-receiver distance (i.e., the distance, d,) plotted on thehorizontal axis was within a range of 0 to 3 mm and that the couplingefficiency approached 100% as the distance, d, was shortened so that thelight receiving element 21 became located closer to the end surface 11of the optical fiber.

As described above, when the plastic optical fiber is used, a commonfiber diameter is from 0.5 mm to 2 mm. Among others, a fiber having adiameter of 1 mm is widely used from the viewpoint of easiness of use,i.e., easiness of connection and suppression of modal dispersion. On theother hand, the transmission rate of high speed communication generallyused for the plastic optical fiber is from 100 Mbps to 622 Mbps, and thePD diameter suitable for that transmission rate is from 0.3 mm to 0.5mm. The combination of the above fiber diameter and the above PDdiameter almost coincides with the range in which the effect of thepresent invention can be produced most.

(Second Embodiment)

FIG. 2 is a schematic view of a second embodiment of the opticalcommunication system of the present invention. The second embodimentdiffers from the first embodiment in that the communication modules 2Aand 2B are each provided with an optical system. In FIG. 2, constituentparts similar to the constituents shown in FIG. 1 are denoted by thesame reference numerals as those shown in FIG. 1. In FIG. 2, referencenumerals 25 and 26 denote a reception optical system and a transmissionoptical system, respectively. The reception optical system 25 isarranged between the light receiving element 21 and a spherical endsurface 11 of the optical fiber 1 and operates to guide the outgoingradiant light from the spherical end surface 11 to the light receivingelement 21. The transmission optical system 26 operates to guide lightemitted from a light emitting element 22 to the other end surface of theoptical fiber 1. The reception optical system 25 and transmissionoptical system 26 include a light refracting member, such as a prism, alens or the like, which is formed of a substance of a refractive indexdifferent from that of air, and a light reflecting member such as amirror or the like. In the present application, when a transparentmolded piece (not shown) or the like of a refractive index differentfrom that of air is formed on the light receiving element 21, even sucha molded piece is treated as a constituent part of the reception opticalsystem 25. The reception and transmission optical systems as describedabove are widely known to those skilled in the art, and therefore, nodetailed description is herein provided therefor. It is acceptable toform a lens portion integrated with a transparent molded piece as thereception optical system 25.

The relational expressions (1) through (4) held between the lightreceiving element 21 and the spherical end surfaces 11 of the opticalfiber 1 in the first embodiment hold between the reception opticalsystem 25 and the spherical end surface 11 of the optical fiber 1 in thesecond embodiment. That is, in the first embodiment the distance fromthe apex of the spherical end surface 11 of the optical fiber 1 to thelight receiving surface of the light receiving element 21 is defined asd by the expressions (1) through (4), while in the second embodiment thedistance from the apex of the spherical end surface 11 of the opticalfiber 1 to the center position of the reception optical system 25 isdefined as d by the expressions (1) through (4). Moreover, although oneof the expressions (1) through (4) is adopted in accordance with thenumerical aperture (NA) of the radiant light emitted from the sphericalend surface 11 of the optical fiber 1 and the diameter (PD diameter) ofthe light receiving element 21 in the first embodiment, the expressions(1) through (4) are applied in accordance with the size of the receptionoptical system 25 instead of the diameter of the light receiving element21 in the second embodiment.

The “center position of the reception optical system 25” is a principalpoint of the reception optical system on the side of incidence of theprincipal ray of light from the optical fiber, as described above.Moreover, the “size of the reception optical system” is a diameter of aportion of the system that optically collect the light when the shape ofthe reception optical system is circular like a condenser lens, and is atypical dimension of a portion of the system that optically collect thelight when the shape is not circular. For example, there is an ovalmirror as the optical system that is not circular, and in this case, theaverage dimension of a section perpendicular to the optical axis at theprincipal point on the incident side of the oval mirror is used as thesize of the optical system.

In the second embodiment as well, the effect of improvement in thereception efficiency similar to that of the first embodiment wasobtained. Utilizing the present invention allows installment of asmall-sized reception optical system by which the light is easilycondensed on a small light receiving element.

(Third Embodiment)

FIG. 20 is a schematic structural view of the optical communicationsystem that adopts a single-core two-way optical communication schemeaccording to a third embodiment of the present invention, and FIG. 21 isan enlarged view of part of FIG. 20. In FIG. 20, constituent elementssimilar to or same as the constituent elements shown in FIGS. 1 and 2are denoted by the same reference numerals as those used in FIGS. 1 and2, and no detailed description is provided therefor.

In contrast to the optical communication systems of the first and secondembodiments in which the one-way communication scheme is adopted and thecooperating two optical communication modules 2A and 2B have either oneof the light receiving element and the light emitting element, each oftwo optical communication modules 2A and 2B that constitute thecommunication system of the third embodiment together with the plasticoptical fiber 1 has both the light emitting element 22 and the lightreceiving element 21 and operate as a transmitter-receiver module. Eachof the optical communication modules 2A and 2B also has a transmissionoptical system 26 and a reception optical system 25. The light receivingand transmitting elements 21 and 22 and the light receiving andtransmitting optical systems 25 and 26 are arranged so that the centerposition of the reception optical system 25 satisfies any one of theexpressions (1) through (4) in accordance with the NA of the outgoinglight from the optical fiber end surface 11 and the size of thereception optical system 25.

It is conceivable to make the light receiving and transmitting elements21 and 22 directly face the optical fiber end surfaces 11 without usingthe optical systems 25 and 26. However, the occupation areas of thelight receiving and transmitting elements 21 and 22 and retentionportions therefor become considerably large with respect to the opticalfiber end surface 11. Therefore, such an arrangement is possible butrather impractical when transmission and reception lights are separatedfrom each other.

Accordingly, when transmission and reception are carried out by oneoptical fiber, an optical system for changing the optical path to thelight receiving element or to the light transmitting element is normallyprovided between the optical fiber end surface and the light receivingand transmitting elements in order to effectively separate thetransmission and reception lights at the small optical fiber endsurface. The light receiving and transmitting optical systems need to besmall-sized in correspondence with the small optical fiber end surface.However, if the reception optical system is excessively small, then theloss of the reception light would be increased if no measure is taken.For example, as shown in FIG. 22, a half of the reception light 15 isdisadvantageously rejected by the transmission optical system 26. If thereception optical system 25 is enlarged as shown in FIG. 23, then thetransmission light 16 may not be coupled with the optical fiber endsurface 11. As shown in FIG. 24, when the light transmitting andreception optical systems are arranged at a great distance from theoptical fiber end surface 11, it is possible to carry out transmissionand reception by using one optical fiber if the NA of the transmissionlight 16 is reduced even when a comparatively large reception opticalsystem 25 is used. However, there would occur a problem that thereception light 15 is disadvantageously spread, making it difficult toachieve coupling to a small PD (light receiving element) 21 adaptable tohigh speed.

However, in the present embodiment, the reception optical system 25 isarranged in the position where any one of the expressions (1) through(4) is satisfied in accordance with the NA of the outgoing light fromthe optical fiber end surface 11 and the size of the reception opticalsystem 25 as described in connection with the second embodiment.Therefore, although the rejection by the transmission optical system 26cannot be helped, the reception optical system 25 efficiently receiveslight even if it has a small size, and is able to guide the receivedlight 15 to the photodiode of the light receiving element 21.

The present invention has been described above on the basis of the threeembodiments, and it is a matter of course that the constructions otherthan the features claimed in the claims, including the materials are notlimited to those described in connection with the above embodiments andare allowed to be subjected to proper alterations and additions.

As is apparent from the above, according to the present invention, evenif a plastic optical fiber having a large aperture is used as atransmission medium and a small-sized light receiving element capable ofcoping with high speed communication is used, an optically high couplingefficiency is obtained with a simple configuration. Particularly, whenthe diameter of the plastic optical fiber is 1 mm, which size is widelyused, and the light receiving element is a small-sized photodiode thathas a diameter of not larger than 0.5 mm and is able to cope with highspeed, the reception efficiency can be effectively increased.

Moreover, the present invention allows the light receiving element andthe reception optical system to have a reduced size. Therefore, whenemploying a single-core two-way optical communication method in whichthe two-way communication is carried out via one optical fiber, thepresent invention is effective from the viewpoint of the arrangement ofthe reception system parallel to the transmission system.

1. An optical communication system comprising: an optical fiber having aspherical end surface at at least one end thereof, wherein radiant lightemitted from the spherical end surface has a numerical aperture of notlarger than 0.35; and an optical communication module which has a lightreceiving element and receives the radiant light emitted from thespherical end surface of the optical fiber, wherein, when the one end ofthe optical fiber is inserted in a prescribed portion inside the opticalcommunication module, a light receiving surface of the light receivingelement is located at a distance, d, from an apex of the spherical endsurface of the optical fiber, and assuming that a diameter of theoptical fiber is D, a radius of curvature, R, of the spherical endsurface is r*D, a refractive index of a core of the optical fiber is n,and a refractive index of a substance that exists between the sphericalend surface of the optical fiber and the light receiving element is n1,then the distance, d, is: within a range of 0<d≦r*D/(n−n1) when adiameter of the light receiving element is not larger than D, and withina range of D≦d≦r*D/(n−n1) when the diameter of the light receivingelement is larger than D.
 2. An optical communication system comprising:an optical fiber having a spherical end surface at at least one endthereof, wherein radiant light emitted from the spherical end surfacehas a numerical aperture of not larger than 0.35; and an opticalcommunication module which has a light receiving element and a receptionoptical system guiding the radiant light emitted from the spherical endsurface of the optical fiber to the light receiving element, andreceives the radiant light emitted from the spherical end surface of theoptical fiber, wherein, when the one end of the optical fiber isinserted in a prescribed portion inside the optical communicationmodule, a center position of the reception optical system is located ata distance, d, from an apex of the spherical end surface of the opticalfiber, and assuming that a diameter of the optical fiber is D, a radiusof curvature, R, of the spherical end surface is r*D, a refractive indexof a core of the optical fiber is n, and a refractive index of asubstance that exists between the spherical end surface of the opticalfiber and the reception optical system is n1, then the distance, d, is:within a range of 0<d≦r*D/(n−n1) when a size of the reception opticalsystem is not larger than D, and within a range of D≦d≦r*D/(n−n1) whenthe size of the reception optical system is larger than D.
 3. An opticalcommunication system comprising: an optical fiber having a spherical endsurface at at least one end thereof, wherein radiant light emitted fromthe spherical end surface has a numerical aperture of 0.4–0.6 inclusive;and an optical communication module which has a light receiving elementand receives the radiant light emitted from the spherical end surface ofthe optical fiber, wherein, when the one end of the optical fiber isinserted in a prescribed portion inside the optical communicationmodule, a light receiving surface of the light receiving element islocated at a distance, d, from an apex of the spherical end surface ofthe optical fiber, and assuming that a diameter of the optical fiber isD, then the distance, d, is: within a range of 0<d<2D when a diameter ofthe light receiving element is not larger than D, and within a range of0.5D<d<2D when the diameter of the light receiving element is largerthan D.
 4. An optical communication system comprising: an optical fiberhaving a spherical end surface at at least one end thereof, whereinradiant light emitted from the spherical end surface has a numericalaperture of 0.4–0.6 inclusive; and an optical communication module whichhas a light receiving element and a reception optical system guiding theradiant light emitted from the spherical end surface of the opticalfiber to the light receiving element, and receives the radiant lightemitted from the spherical end surface of the optical fiber, wherein,when the one end of the optical fiber is inserted in a prescribedportion inside the optical communication module, a center position ofthe reception optical system is located at a distance, d, from an apexof the spherical end surface of the optical fiber, and assuming that adiameter of the optical fiber is D, then the distance, d, is: within arange of 0<d<2D when a size of the reception optical system is notlarger than D, and within a range of 0.5D<d<2D when the size of thereception optical system is larger than D.
 5. The optical communicationsystem as claimed in claim 1, wherein the optical fiber is a plasticoptical fiber.
 6. The optical communication system as claimed in claim1, wherein the substance is air whose refractive index n1 is one.
 7. Theoptical communication system as claimed in claim 1, wherein thediameter, D, of the optical fiber is 1 mm, and the light receivingelement is a photodiode that has a diameter of not larger than 0.5 mm.8. The optical communication system as claimed in claim 3, wherein thediameter, D, of the optical fiber is 1 mm, and the reception opticalsystem has a size of not larger than 0.5 mm.
 9. The opticalcommunication system as claimed in claim 1, wherein the opticalcommunication module further comprises, of a light emitting element anda transmission optical system, at least the light emitting element suchthat the optical communication module is able to transmit and receive asignal light via the optical fiber to and from a counterpart opticalcommunication module in a single-core two-way communication scheme. 10.The optical communication system as claimed in claim 2, wherein theoptical fiber is a plastic optical fiber.
 11. The optical communicationsystem as claimed in claim 2, wherein the substance is air whoserefractive index n1 is one.
 12. The optical communication system asclaimed in claim 2, wherein the diameter, D, of the optical fiber is 1mm, and the light receiving element is a photodiode that has a diameterof not larger than 0.5 mm.
 13. The optical communication system asclaimed in claim 2, wherein the optical communication module furthercomprises, of a light emitting element and a transmission opticalsystem, at least the light emitting element such that the opticalcommunication module is able to transmit and receive a signal light viathe optical fiber to and from a counterpart optical communication modulein a single-core two-way communication scheme.
 14. The opticalcommunication system as claimed in claim 3, wherein the optical fiber isa plastic optical fiber.
 15. The optical communication system as claimedin claim 3, wherein the diameter, D, of the optical fiber is 1 mm, andthe light receiving element is a photodiode that has a diameter of notlarger than 0.5 mm.
 16. The optical communication system as claimed inclaim 3, wherein the optical communication module further comprises, ofa light emitting element and a transmission optical system, at least thelight emitting element such that the optical communication module isable to transmit and receive a signal light via the optical fiber to andfrom a counterpart optical communication module in a single-core two-waycommunication scheme.
 17. The optical communication system as claimed inclaim 4, wherein the optical fiber is a plastic optical fiber.
 18. Theoptical communication system as claimed in claim 1, wherein thediameter, D, of the optical fiber is 1 mm, and the light receivingelement is a photodiode that has a diameter of not larger than 0.5 mm.19. The optical communication system as claimed in claim 4, wherein thediameter, D, of the optical fiber is 1 mm, and the reception opticalsystem has a size of not larger than 0.5 mm.
 20. The opticalcommunication system as claimed in claim 4, wherein the opticalcommunication module further comprises, of a light emitting element anda transmission optical system, at least the light emitting element suchthat the optical communication module is able to transmit and receive asignal light via the optical fiber to and from a counterpart opticalcommunication module in a single-core two-way communication scheme.